1. Field of the Invention
The present invention generally relates to the information processing technology. More particularly, the present invention relates to an information processing technique of using design data for expressing the profile defined for an object to be examined and point group data obtained by actually measuring the profile of the object.
2. Description of the Related Art
In the process of designing a product, it is a general practice to use a CAD apparatus (2D-CAD apparatus or 3D-CAD apparatus) in order to design an object having a three-dimensional profile (to be referred to simply as “part” hereinafter). The designer of a part conveys design/manufacture information and the design purposes of the part to downstream steps typically including a manufacturing step by inputting attribute information on the part such as dimensions, dimensional tolerances, geometric tolerances, footnotes and signs on a CAD model, which may be a 3D model or 2D drawings.
The downstream steps that may typically include a step of designing a metal mold confirm the design information, referring to the CAD model from the upstream side. After the confirmation, the metal mold, the NC programming, the metal mold manufacture, the molding and the molded products or the metal mold or molds will be inspected.
In the inspection step, a measurement operation is conducted, based on the design information such as dimensional tolerances directed typically in drawings, by means of a contact type coordinate measuring machine (CMM), a non-contact type CMM or a manual measuring machine such as a microscope. It is a general practice that an identifier is affixed to each dimension to record measured values with their respective identifiers so that the measured results can be verified on the drawings.
For such inspections, Japanese Patent Application Laid-Open No. H08-082575 discloses a technique for improving the efficiency of measurement operations by automating the process of totalizing the results of measurement. Japanese Patent Application Laid-Open No. H08-190575 and Japanese Patent Application Laid-Open No. 2000-235594 describe other techniques of the same category. According to these patent documents, the measurement values are output with routing data of the CMM, to which the identifiers of dimensions are added, and the results of measurement are read into the CAD apparatus and the dimensions are compared with the dimensions on the CAD drawings for collation. Such techniques are adopted for a commercially available CAD system.
On the other hand, Japanese Patent Application Laid-Open No. 2001-82951 proposes a method of aligning a CAD model and corresponding point group data obtained from a non-contact type CMM. Recently, pieces of software for interactively displaying the quantity of discrepancy between CAD data and corresponding point group data and then displaying the point group with a color that varies as a function of the quantity of discrepancy have been marketed.
However, it is necessary to prepare a measurement program for each site of measurement when utilizing an automatic measuring machine such as a CMM. Additionally, it is time consuming to use a manual measurement machine. Particularly, when the number of sites of measurement is large, it may sometimes take several days for the measurements to become an obstacle of development of a product. Thus, when a contact type measurement technique is employed, the measurement operation itself is time consuming. Then, it is normally difficult to satisfy both the requirements of cost and those of delivery date simply by automating the operation of processing measurement data as disclosed in the above cited patent documents including Japanese Patent Application Laid-Open No. H08-082575, Japanese Patent Application Laid-Open No. H08-190575 and Japanese Patent Application Laid-Open No. 2000-235594.
On the other hand, it is possible to reduce the time required for measurement by employing a technique of comparing the point group data obtained from the profile of a part by means of a non-contact type measurement machine and the corresponding CAD model for collation. With such a technique, the results of measurement of a part are evaluated relative to the attribute information such as dimensions of the part affixed to the CAD model. For this purpose, a cross section of the point group data that is aligned with the CAD model is taken for the measurement or faces and edges are prepared from the point group data and some of the faces and the edges are interactively selected for measurement. Then again, it is time consuming to evaluate the results of measurement on a dimension by dimension basis.
A technique disclosed in Japanese Patent Application Laid-Open No. 2001-82951 utilizes the distance between point group data and profile elements such as dimensions when associating them with each other. However, in general, the profile of a part to be measured shows errors relative to the profile of the CAD drawings. Therefore, with a technique of simply utilizing the distance between CAD drawings and a point group, the operation of associating the point group with the profile elements can involve errors. If errors are involved in the operation of associating them with each other, it is consequently difficult to conduct an accurate inspection.
Additionally, some profile elements may be unsuitable for measurement because they show a large error relative to the corresponding point group. It is difficult to give an appropriate determination when the determination is not made at a site suitable for measurement or at a measurement point that reflects the designer's intention.
In view of the above-identified problems, it is therefore the object of the present invention to provide a technique for preventing errors from arising when associating actual measurement data of a part with model data.